1. Field of the Invention
The present invention relates to a technique for compensating for the effects of an environmental magnetic field, particularly geomagnetism, in which a device is placed. More particularly, the invention relates to a geomagnetism compensating technique suitable for a high-resolution CRT display device for use as a computer display device.
2. Description of the Background Art
High-resolution CRT display devices for use as, e.g., computer display devices with 17- to 21-inch diagonal screens and with 1280-dot by 1024-line resolution have become predominant. Further, such devices with 22- to 24-inch diagonal screens have been increasingly required to provide a 1600-dot by 1200-line resolution. To meet the requirement for such a resolution, the currently prevailing pitch of phosphors is 0.28 mm. However, there is also a need for high-definition CRTs with a phosphor pitch as fine as 0.25 mm.
Unfortunately, such high-resolution computer CRT display devices are in some cases affected by geomagnetism, e.g. the vertical component thereof, to generate variations in horizontal image position, convergence and beam landing, causing degradation in image quality of the devices. In particular, the above-mentioned high-definition CRTs with fine phosphor pitch exhibit more profound effects of the same amount of horizontal image position variation, the same amount of misconvergence and the same amount of beam mislanding upon the degradation in device image quality than do CRTs with larger phosphor pitches, to show the adverse effects of geomagnetism, e.g. the vertical component thereof, which appear more significantly.
The following techniques have been proposed to prevent variations in horizontal image position, convergence and beam landing under the influence of such magnetism, e.g. the vertical component thereof:
(1) To prepare exposure designs separately for CRTs for use in the Northern Hemisphere and CRTs for use in the Southern Hemisphere since geomagnetism, e.g. the vertical component thereof, differs greatly between the Northern Hemisphere and the Southern Hemisphere on Earth.
(2) To enhance magnetic shielding to reduce the effects of geomagnetism, e.g. the vertical component thereof.
(3) To provide means for correcting the variations in horizontal image position, convergence and beam landing under the influence of geomagnetism, e.g. the vertical component thereof.
The technique (1) which prepares the separate CRT exposure designs increases costs and is therefore impractical. The technique (2) which merely enhances the magnetic shielding is insufficient to solve the problem. Hence, the technique (3) for correcting the horizontal image position variation, misconvergence and beam mislanding by using the correcting means has been examined.
FIG. 25 is a perspective view illustrating the correcting means of a CRT 16. The CRT 16 comprises a deflection yoke 13 serving as a fundamental part, and a convergence correction coil 14 and a beam landing correction coil 15 which are mounted around the neck thereof. The convergence correction coil 14 and the beam landing correction coil 15 are provided to correct misconvergence and beam mislanding, respectively. Respective correction currents are supplied to the deflection yoke 13 and the correction coils 14 and 15.
FIG. 26 is a block diagram of a circuit for supplying the correction currents. The circuit of FIG. 26 comprises a first individual adjustment means 17 which sets standard adjustment values for correction of the horizontal image position, misconvergence and beam mislanding, for example, during the manufacture of a CRT display device, and a second individual adjustment means 18 for correcting the variations in horizontal image position, convergence and beam landing under the influence of geomagnetism, e.g. the vertical component thereof, at the position of installation, for example, when the CRT display device is installed.
Each of the first and second individual adjustment means 17 and 18 supplies three correction current adjustment values to adder-subtracter circuits 19 to 21, respectively. The adder-subtracter circuits 19 to 21 perform addition and subtraction upon the adjustment values from the first and second individual adjustment means 17 and 18. The results of addition and subtraction from the adder-subtracter circuits 19 to 21 are supplied to drive circuits 22 to 24 for driving the deflection yoke 13, the convergence correction coil 14 and the beam landing correction coil 15, respectively. Thus, the correction currents in accordance with the adjustment values are supplied to the deflection yoke 13, the convergence correction coil 14 and the beam landing correction coil 15, respectively.
The geomagnetism-related correction using the circuit shown in FIG. 26 is required to adjust the second individual adjustment means 18 when the CRT 16 is installed, moved or changed in orientation thereof. However, such an adjustment requires special measuring equipment and expert knowledge for adjusting the horizontal image position, convergence and beam landing while making a measurement on a display screen of the CRT 16. Therefore, the technique (3) is disadvantageous in that a user that makes the adjustment by himself or herself finds difficulty in accomplishing a successful result and in that the installation or movement of the CRT display device requires a complicated procedure such as the visit of a serviceman having expert knowledge. Another disadvantage is that the above-mentioned correction circuit constructed in hardware form has a very complicated and large-scale circuit configuration.
According to a first aspect of the present invention, an environmental magnetism compensating device comprises: a magnetism sensor for detecting a vertical component of a magnetic environment in which a cathode-ray tube including a deflection yoke, a convergence correction coil and a beam landing correction coil is placed to output a detection signal; an arithmetic unit for determining first to third parameters based on the detection signal; and a driver for supplying current having values set based on the first to third parameters, respectively, to the deflection yoke, the convergence correction coil and the beam landing correction coil.
Preferably, according to a third aspect of the present invention, in the environmental magnetism compensating device of the first aspect, the current supplied to the convergence correction coil varies at two different rates of change for a time period corresponding to one frame in synchronism with a vertical deflection signal for the cathode-ray tube.
Preferably, according to a third aspect of the present invention, in the environmental magnetism compensating device of the first aspect, the current supplied to the convergence correction coil varies at two difference rates of change for a time period corresponding to one frame in synchronism with a vertical deflection signal for the cathode-ray tube.
Preferably, according to a fourth aspect of the present invention, in the environmental magnetism compensating device of the first aspect, the current supplied to the beam landing correction coil is in synchronism with a vertical deflection signal for the cathode-ray tube and has a waveform symmetrical with respect to a midpoint of a time period corresponding to one frame.
Preferably, according to a fifth aspect of the present invention, in the environmental magnetism compensating device of the first aspect, the current supplied to the beam landing correction coil is in synchronism with a vertical deflection signal for the cathode-ray tube and has a waveform asymmetrical with respect to a midpoint of a time period corresponding to one frame.
Preferably, according to a sixth aspect of the present invention, in the environmental magnetism compensating device of the fourth or fifth aspect, the current supplied to the beam landing correction coil has a variable DC level.
According to a seventh aspect of the present invention, a cathode-ray tube display device comprises: an environmental magnetism compensating device as recited in any one of the first to sixth aspects; and the cathode-ray tube.
In accordance with the environmental magnetism compensating device of the first aspect of the present invention, the values of current supplied to the deflection yoke for correcting a horizontal image position, the convergence correction coil for correcting misconvergence and the beam landing correction coil for correcting beam mislanding are set respectively based on the first to third parameters determined based on the vertical component of the magnetic environment. If the magnetic environment in which the CRT is placed is changed when the CRT is installed, moved or changed in orientation thereof, the arithmetic unit determines the first to third parameters suitable for the changed magnetic environment. Therefore, the environmental magnetism compensating device of the first aspect can make an automatic adjustment which corrects the variations in horizontal image position, convergence and beam landing without the need for special measuring equipment, expert knowledge and user""s adjustment.
The environmental magnetism compensating device of the second aspect of the present invention can readily correct such misconvergence that the directions thereof on upper and lower parts of the CRT are opposite from each other under the influence of the vertical component of the magnetic environment.
The environmental magnetism compensating device of the third aspect of the present invention can correct misconvergence when the amount of misconvergence resulting from the vertical component of the magnetic environment differs between the upper and lower parts of the CRT, for example, due to the deviation of the position of electron guns.
The environmental magnetism compensating device of the fourth aspect of the present invention can readily correct beam mislanding which tends to decrease in upward and downward directions from the center of the CRT under the influence of the vertical component of the magnetic environment.
The environmental magnetism compensating device of the fifth aspect of the present invention can correct beam mislanding when the amount of beam mislanding resulting from the vertical component of the magnetic environment differs between the upper and lower parts of the CRT, for example, due to the deviation of the position of electron guns.
The environmental magnetism compensating device of the sixth aspect of the present invention can counteract the influence of variations due to individual differences in CRT which include variations in the beam landing correction coil when the device corrects beam mislanding resulting from the vertical component of the magnetic environment.
The cathode-ray tube display device of the seventh aspect of the present invention may be installed, moved and changed in orientation thereof without consideration of variations in horizontal image position, convergence and beam landing resulting from variations in the magnetic environment.
It is therefore an object of the present invention to provide a technique for making an automatic adjustment which corrects variations in horizontal image position, convergence and beam landing under the influence of geomagnetism, e.g. the vertical component thereof, when a CRT display device employing a high-definition CRT is installed or moved without the need for special measuring equipment, expert knowledge and user""s adjustment.